5 The light curve

Figure 7: The visual lightcurve of the outburst. The light curve of the 1917 outburst determined by Williams (2000) is shown for comparison. The maximum occured on May 5.0, 2000 (JD 2451669.5)

In order to estimate the light curve parameters (epoch of maximum, rates of decline), we used the CI Aql record in the VSNET archive (this data also includes the observations published in the IAU Circulars). It contains 2309 individual data points between April 28 and August 16, 2000 (1737 CCD observations and 572 visual estimates). We plot the light curve in Fig. 7. The epoch of maximum was determined by fitting a low-order polynomial around the top of the light curve, while $t_{\rm 2}$ and $t_{\rm 3}$were read off from the smoothed light curve (see Fig. 7). The derived parameters are: $t_{\rm0}=2451669.5 \pm 0.1$ (2000 May 5.0 UT), $t_{\rm 2}=30\pm1$days, $t_{\rm 3}=36\pm1$ days. Consequently, CI Aql is a moderately fast nova. The apparent magnitudes in maximum (9 $.\!\!^{\rm m}$0) and minimum (16 mag) give a considerably low outburst amplitude of $\sim$7 mag. We plotted also the light curve of the 1917 outburst determined by Williams (2000) in Fig. 7. The similarity is evident, though a significant deviation is suggested at $\Delta t$ = +20 d, when an additional brightening occured in 2000. The maxima were at similar brightness, but later differences as high as 2 mag can be found. Recent $t_{\rm 2}$ (30 d) differs significantly from that of in 1917 (18 d) which is entirely due to a plateau of the light curve of the 2000 outburst. Consequently, $t_{\rm 3}$appears to be a more reliable indicator of the rate of decline.

Three maximum magnitude versus rate of decline (MMRD) relations were used to calculate visual absolute magnitude (Della Valle & Livio 1995; Capaccioli et al. 1989; Schmidt 1957). They result in -7 $.\!\!^{\rm m}$4, -7 $.\!\!^{\rm m}$3 and -7 $.\!\!^{\rm m}$6. The constant absolute magnitude 15 days after the maximum (Capaccioli et al. 1989) gives -7 $.\!\!^{\rm m}$1. Their simple mean is $M_V=-7\hbox{$.\!\!^{\rm m}$ }35\pm0\hbox{$.\!\!^{\rm m}$ }2$ (formal error). However, the true uncertainty could be much larger as even the applicability of MMRD relations for the RN outburst can be questionned (e.g. the outbursts of U Sco are regularly underluminous compared to those of classical novae, Munari et al. 1999). Any meaningful luminosity value would need proper modelling of either the outburst or observations carried out in the quiescence, which is beyond the scope of this paper.